# Guitar Tube Amp RC Filter frequency fluctuation

In a simple LP or HP RC filter, is there a scenario where the centering frequency can fluctuate? So say the filter is a low pass and its set to pass everything up to 800hz, is there a situation where it may cut off at 750hz, or perhaps at a different time cut off at 825hz, etc.?

Maybe the guitarist picks the strings harder/softer, air pressure, ambient temperature, etc. could cause this? I am researching simulation techniques and trying to understand if factors like these could make an amp model a little more realistic in terms of tonal quality and dynamics.

The "Feel" of an amp plays a major role in defining the characteristics of how one plays and changing frequencies that are allowed to pass into a non-linear function will alter the feel quite drastically (at least in the digital amp sim world). The difference could be relatively small too, 120hz changed to 100hz will add a more "saturated" type feel. You may not notice much difference in tone, but you will in "feel". In the analog world I have noticed this "difference" in cut off only added to tone(allowed for more bass), but had little effect on feel.

So, can (or do) RC Filter cut off frequencies fluctuate?

Thank you..

A low pass filter is pretty much defined by the R and the C value: -

$f_C = \dfrac{1}{2\pi R C}$

The same applies for a simple RC high pass filter and the steepness of the transition from "pas" to "reject" frequencies is very shallow: -

Even if the cut-off frequency did fluctuate by (say) 1% (and likely very slowly due to warming and cooling effects of the capacitor) the perceived change in tone would be minimal over that long period of time (at least several minutes.

Think about a slow 1dB change in volume - this could occur in a space of 2 seconds and you just wouldn't notice it let alone over a period greater than several minutes.

And there is certainly no dynamic effect of tone shifting due to the way the guitarist picks or strums the strings.

I don't think looking at CR filter changes is going to yield results that you think. I play and record guitar and have done so over many years. I do all sorts of things to the recorded guitar sound from basic equalization (messing with tone controls) to complicated effects and the least clearly obvious things that effect the sound of a guitar is movement of tone unless it is something dramatic like the wah-wah effect.

• PART 1: Wow, fantastic Andy, thank you so much for your insight. So these fluctuations are not the golden goose I'm looking for, they move too slowly. I notice with some sims, after hitting a chord with a heavily distorted signal, as it fades out, bass frequencies become stronger. This differs from a real amp. Jun 4, 2014 at 3:29
• PART 2: So I am trying to figure out if that happens because the signal is fading and as such it develops less harmonics (less high frequencies as a result of less distortion), or if it is something in the real amp that does this? Jun 4, 2014 at 3:30
• PART 3: It does not seem to have anything to do with aliasing, in that it still happens even at a sample rate of 768,000 (96k oversampled 8x). Also there is an anti-aliasing filter on it as well set to 19k. So clearly, no frequencies are even getting close to nyquist. Well once again, thank you Jun 4, 2014 at 3:31
• When using distortion, as the signal fades the higher frequency harmonics (due to distortion) reduce at a faster rate than the fundamental frequency. It's nothing to do with aliasing or sampling - this is natural when using distortion because it clips the waveform. Jun 4, 2014 at 7:13

"So, can (or do) RC Filter cut off frequencies fluctuate?"

In principle - yes. But, for example due to temperature influence perhaps by +/- 0.2%. However, are you aware that the cut-off frequency of such a passive filter can/will be influenced by any load resp. load change? Or did you use a decoupling buffer?

• Thank you for your reply, and in answer to your question. No I have not made a hardware version of this at all. I am on a fact finding (research) mission. I am interested in areas where digital amp simulation differs from analog amps. Simulators seem quite static and it is things like these fluctuations that may make a difference.. :) Thank you oncde again.. Jun 4, 2014 at 3:16